Method for processing fischer-tropsch off-gas

ABSTRACT

This invention concerns a method for recovering carbon monoxide and carbon dioxide from Fischer-Tropsch off-gas by feeding Fischer-Tropsch off-gas through a column comprising an adsorbent bed, and discharging effluent, optionally rinsing the column and the adsorbent bed by feeding NG and discharging effluent until at least 60% of the carbon monoxide that was present in the bed is discharged, pressurizing the column and adsorbent bed with NG, rinsing the column and the adsorbent bed by feeding NG until at least 50% of the carbon dioxide present at the commencement of this rinsing step is discharged, rinsing the column and adsorbent bed by feeding a mixture of hydrogen and nitrogen, pressurizing the column and adsorbent bed by feeding a mixture of hydrogen and nitrogen. With this method a feed comprising at least 50 vol % carbon monoxide can be produced. Furthermore, methane and carbon dioxide at a high pressure can be recovered from the Fischer-Tropsch gas. This can be fed to a gasifier or a reformer. In a preferred embodiment a gas comprising at least 80 vol % hydrogen is produced as well.

FIELD OF THE INVENTION

The present invention relates to a process for processingFischer-Tropsch off-gas. Carbon dioxide and carbon monoxide, andoptionally hydrogen, can be recovered from the off-gas in a veryefficient way.

BACKGROUND OF THE INVENTION

The Fischer-Tropsch process can be used for the conversion ofhydrocarbonaceous feed stocks into normally liquid and/or solidhydrocarbons (i.e. measured at 0° C., 1 bar). The feed stock (e.g.natural gas, associated gas, coal-bed methane, residual oil fractions,biomass and/or coal) is converted in a first step into a mixture ofhydrogen and carbon monoxide. This mixture is often referred to assynthesis gas or syngas. The synthesis gas is fed into a reactor whereit is converted over a suitable catalyst at elevated temperature andpressure into paraffinic compounds ranging from methane to highmolecular weight molecules comprising up to 200 carbon atoms, or, underparticular circumstances, even more.

The hydrocarbon products manufactured in the Fischer-Tropsch process areprocessed into different fractions, for example a liquid hydrocarbonstream comprising mainly C₅+ hydrocarbons, and a gaseous hydrocarbonstream which comprises methane, carbon dioxide, unconverted carbonmonoxide, unconverted hydrogen, and lower hydrocarbons. The gaseoushydrocarbon stream may also comprise nitrogen as the syngas sent to theFischer-Tropsch reactor may contain some nitrogen.

The gaseous hydrocarbon stream is often referred to as Fischer-Tropschoff-gas. Fischer-Tropsch off-gas can be recycled to the syngasmanufacturing or to the Fischer-Tropsch reactor. Sometimes lowerhydrocarbons are removed before the off-gas is recycled. Lowerhydrocarbons may be removed by decreasing the temperature of the off-gasand then applying a gas-liquid separation. However, when the off-gas isrecycled to the syngas manufacturing or to the Fischer-Tropsch reactor,the components in the off-gas which do not take part in theFischer-Tropsch reaction, such as carbon dioxide, nitrogen and methane,occupy reactor space. The components which do not take part in theFischer-Tropsch reaction are also referred to as “inerts”.

The level of inerts in the Fischer-Tropsch reactor increases withincreasing Fischer-Tropsch off-gas recycling. The pace of the build-upof inerts can be reduced by treating the off-gas before it is recycled.When the off-gas is passed through a pressure swing adsorption unit(PSA), it is normally possible to remove carbon dioxide and water fromthe off-gas. It is often possible to recover a hydrogen stream from theoff-gas by means of a PSA unit; the hydrogen stream can be recycled tothe Fischer-Tropsch reactor. Nevertheless, common commercial PSA unitsare often not designed to recover a carbon monoxide stream. And somecommon commercial PSA units result in a hydrogen stream comprising asignificant amount of nitrogen. Therefore it is common to recycle only arelatively small part of the off-gas. One possibility is to recycle apart of the Fischer-Tropsch off-gas to one or more Fischer-Tropschreactors while another part of the off-gas is used as fuel. A downsideof this is that only a part of the carbon atoms of the hydrocarbonaceousfeed stock is converted to the desired C₅+ hydrocarbons.

U.S. Pat. No. 5,112,590 and U.S. Pat. No. 5,096,470 describe theseparation of gases using specific PSA systems with a first PSA unit forproducing hydrogen and a second PSA unit for producing carbon monoxide.Such systems may be useful for gas mixtures comprising a relatively highamount of hydrogen. Such systems are, however, not suitable for gasmixtures comprising a relatively low amount of hydrogen, e.g. less than50 volume % calculated on the total gas mixture. Furthermore, in case ofa gas feed comprising a significant amount of nitrogen such systems willnot result in the product cuts as described. When, for example, purehydrogen would be separated using the first PSA, nitrogen wouldcontaminate the intermediate carbon monoxide stream in a systemaccording to U.S. Pat. No. 5,112,590 or U.S. Pat. No. 5,096,470. Thesystems of U.S. Pat. No. 5,112,590 and U.S. Pat. No. 5,096,470 maytherefore be suitable to treat a hydrogen-rich gas mixture exiting asteam methane reformer, but they are not suitable to treat anitrogen-comprising hydrogen-lean off-gas of a Fischer-Tropsch process.

US20110011128 describes a PSA comprising system in which purifiedhydrogen is produced using a PSA, which may be a conventional co-purgeH₂ PSA unit. Such a system may be useful to a hydrogen-rich gas mixtureexiting a steam methane reformer, but is not suitable to treat nitrogencomprising hydrogen-lean off-gas of a Fischer-Tropsch process.

US20040077736 mentions a process in which a liquid phase and a vapourphase are withdrawn from a hydrocarbons synthesis stage. In a vapourphase work-up stage, hydrocarbon products having 3 or more carbon atomsmay be removed and the residual vapour phase may then pass to a PSA.Using the PSA first, second and optionally third gas components areseparated. The first gas component comprises carbon monoxide andhydrogen. The second gas component comprises methane, and the optionalthird gas component comprises carbon dioxide. The first gas component isrecycled to the hydrocarbon synthesis stage. US20040077736 does notprovide details on the method PSA method used. A regular use of a normalPSA would result in a relatively low recovery of carbon monoxide in thefirst gas component, and a build-up of nitrogen in the reactor uponrecycling the first gas component to the hydrocarbon synthesis stage.

US20080300326-A1 describes the use of a PSA method to separateFischer-Tropsch off-gas. The method produces at least one gas streamcomprising hydrogen, at least one gas stream mainly comprising methane,and at least one gas stream comprising carbon dioxide, nitrogen and/orargon, and hydrocarbons with at least 2 carbon atoms. The PSA usedcomprises at least three adsorbent beds: alumina, carbon molecularsieves or silicates, activated carbon, and optionally zeolite. Thealumina is used to remove water. The carbon molecular sieves orsilicates are used to adsorb carbon dioxide and partially methane. Theactivated carbon is used to adsorb methane and partially nitrogen andcarbon monoxide. Zeolite may be used to adsorb nitrogen, argon andcarbon monoxide. The product stream of the PSA mainly compriseshydrogen. The other gas streams are obtained during the decompressionphase. Disadvantages of the method of US20080300326-A1 are at least thefollowing. Nitrogen is only partially adsorbed in the PSA. This resultsin a build-up of nitrogen in the Fischer-Tropsch reactor when thehydrogen stream is used, i.e. recycled, as reactant gas. Also themethane stream comprises nitrogen and thus results in the build-up ofnitrogen in the syngas, and thus in the Fischer-Tropsch reactor, whenthe methane stream is used for generating syngas. Another disadvantageof the method of US20080300326-A1 is that carbon monoxide is onlyrecycled to the Fischer-Tropsch reactor in a limited amount. Carbonmonoxide is present in the hydrogen stream and in the methane stream.Nevertheless, at least 50% of the CO initially present in the off-gasends up in the third stream which is used as fuel.

There is a desire to recover both carbon dioxide and carbon monoxidefrom Fischer-Tropsch off-gas in an efficient way. The carbon dioxide andcarbon monoxide could then, together or separately, be recycled to oneor more units in a Fischer-Tropsch line-up. This would make it possibleto convert most of the carbon atoms of the hydrocarbonaceous feed stockto the desired C₅+ hydrocarbons. It is even more desired to additionallyobtain a pure hydrogen stream from Fischer-Tropsch off-gas, which may berecycled to the Fischer-Tropsch reactor.

SUMMARY OF THE INVENTION

The invention provides a method for recovering carbon monoxide andcarbon dioxide from a gas mixture comprising 5-50 vol % methane, 10-50vol % carbon dioxide, 20-65 vol % carbon monoxide, 10-40 vol % hydrogenand 10-55 vol % nitrogen, calculated on the total volume of the gasmixture, said method comprising, in sequence, the following steps:

-   (1) feeding a gas mixture through a column comprising an adsorbent    bed,    -   said gas mixture comprising 5-50 vol % methane, 10-50 vol %        carbon dioxide, 20-65 vol % carbon monoxide, 10-40 vol %        hydrogen and 10-55 vol % nitrogen, calculated on the total        volume of the gas mixture,    -   the adsorbent bed comprising alumina, a carbon molecular sieve,        silicalite, activated carbon, a zeolite, or mixtures thereof,        -   with upon commencement of said feeding, the bed and column            being pre-saturated and pre-pressurized to a pressure in the            range of 10 to 80 bar absolute (bar a), preferably 20 to 70            bar a, with pure hydrogen, or with a mixture of hydrogen and            nitrogen, and        -   discharging effluent from the other end of said bed, and        -   continuing said feeding and said discharging until a carbon            monoxide comprising gas has reached at least 45% of the            length of the bed and has reached at most 80% of the length            of the bed, calculated from the end of the bed at which the            gas mixture is being fed; and-   (2) ceasing the feeding of the gas mixture comprising 5-50 vol %    methane, 10-50 vol % carbon dioxide, 20-65 vol % carbon monoxide,    10-40 vol % hydrogen and 10-55 vol % nitrogen, and reducing the    pressure in the column and the bed by about 5 to 25 bar a; and-   (3) optionally rinsing the column and the adsorbent bed by feeding a    gas comprising at least 70 vol % methane through the column and    adsorbent bed,    -   the column and bed being at a pressure in the range of 5 to 75        bar a, preferably 25 to 65 bar a, more preferably 30 to 55 bar        a, and    -   discharging effluent from the other end of said bed, and    -   continuing said feeding and said discharging until at least 60%        of the carbon monoxide that was present in the bed at the        commencement of this rinsing step is discharged from the other        end of said bed;-   (4) pressurizing the column and adsorbent bed by about 5 to 25 bar a    by feeding a gas comprising at least 70 vol % methane,-   (5) rinsing the column and the adsorbent bed by feeding a gas    comprising at least 70 vol % methane through the column and    adsorbent bed,    -   the column and bed being at a pressure in the range of 30 to 80        bar a, preferably 30 to 70 bar a, and    -   discharging effluent from the other end of said bed, and    -   continuing said feeding and said discharging until an amount        equal to at least 50% of the carbon dioxide that was present at        the commencement of this rinsing step is discharged from the        other end of said bed; and-   (6) ceasing the feeding of a gas comprising at least 70 vol %    methane and reducing the pressure of the column and adsorbent bed to    a pressure in the range of 15 to 25 bar a; and-   (7) further reducing the pressure of the column and adsorbent bed to    a pressure in the range of 1 to 5 bar a; and-   (8) rinsing the column and adsorbent bed by feeding a mixture of    hydrogen and nitrogen through the column and adsorbent bed    -   the column and bed being at a pressure in the range of 1 to 5        bar a, and-   (9) pressurizing the column and adsorbent bed to a pressure in the    range of 15 to 75 bar a, preferably 25 to 65 bar a, more preferably    30 to 55 bar a by feeding a mixture of hydrogen and nitrogen,    whereby said method optionally further comprises the following    steps:-   (A) feeding the effluent of step (1) through a column comprising an    adsorbent bed, the adsorbent bed comprising alumina, a carbon    molecular sieve, silicalite, activated carbon, a zeolite, or    mixtures thereof,    -   with upon commencement of said feeding, the bed and column being        pre-saturated and pre-pressurized to a pressure in the range of        20 to 80 bar absolute (bar a), preferably 30 to 70 bar a, with a        gas comprising more than 95 volume % hydrogen, preferably a gas        comprising 90 to 99.9 volume % hydrogen, and    -   discharging effluent from the other end of said bed, and    -   continuing said feeding and said discharging until a nitrogen        comprising gas has reached at least 45% of the length of the bed        and has reached at most 80% of the length of the bed, calculated        from the end of the bed at which the gas mixture is being fed;        and-   (B) ceasing the feeding of the effluent of step (1), and reducing    the pressure in the column and the bed by about 2 to 5 bar a; and-   (C) further reducing the pressure in the column and the bed by about    2 to 5 bar a; and-   (D) reducing the pressure of the column and adsorbent bed to a    pressure in the range of 1 to 5 bar a; and-   (E) rinsing the column and adsorbent bed by feeding a gas comprising    more than 95 volume % hydrogen, preferably a gas comprising 90 to    99.9 volume % hydrogen, through the column and adsorbent bed    -   the column and bed being at a pressure in the range of 1 to 5        bar a, and-   (F) pressurizing the column and adsorbent bed to a pressure in the    range of 5 to 50 bar a, preferably 10 to 45 bar a, more preferably    20 to 40 bar a by feeding a gas comprising more than 95 volume %    hydrogen, preferably a gas comprising 90 to 99.9 volume % hydrogen,    and-   (G) further pressurizing the column and adsorbent bed to a pressure    in the range of 15 to 75 bar a, preferably 25 to 65 bar a, more    preferably 30 to 55 bar a by feeding a gas comprising more than 95    volume % hydrogen, preferably a gas comprising 90 to 99.9 volume %    hydrogen.

One advantage of the present invention is that the method is suitable toobtain a carbon-monoxide rich stream from Fischer-Tropsch off-gas inoptional step (3). This carbon-monoxide rich stream can be recycled tothe Fischer-Tropsch reactor. A recovery of CO of greater than 50% may beobtained.

The process is also suitable to recover unconverted hydrogen in anefficient way from the Fischer-Tropsch off-gas. Unconverted hydrogen maybe recycled to a Fischer-Tropsch reactor.

Another advantage of the present invention is that the method issuitable to recover a gas comprising methane and carbon dioxide, andoptionally carbon monoxide, from the Fischer-Tropsch off-gas in step(5), whereby this gas can be obtained at high pressure. When the gascomprising carbon dioxide is recovered at a high pressure it can be fedto a gasifier or a reformer, for example an auto thermal reformer,without performing a recompression step.

The process of the present invention makes it possible to convert mostof the carbon atoms of the hydrocarbonaceous feed stock to the desiredC₅+ hydrocarbons.

The invention is further illustrated in the accompanying drawing.

DRAWING

FIG. 1 shows an overview of the process steps of a preferred methodaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to processing of off-gas obtained from aFischer-Tropsch reactor in order to convert most of the carbon atoms ofthe hydrocarbonaceous feed stock to the desired C₅+ hydrocarbons. Whenremoved from a Fischer-Tropsch reactor, the Fischer-Tropsch off-gas isgenerally at a temperature in the range of 40-100° C., preferably in therange of 50-70° C. and at a pressure of 20-80 bar, preferably in therange of 50-70 bar.

Fischer-Tropsch off-gas is typically produced by a Fischer-Tropschhydrocarbon synthesis process comprising the steps of:

i) conversion of a (gaseous) hydrocarbonaceous feed to obtain synthesisgas (syngas);ii) catalytic conversion of the synthesis gas obtained in step i) usinga Fischer-Tropsch catalyst into a Fischer-Tropsch product; andiii) separating the Fischer-Tropsch product of step ii) into at leastone hydrocarbon product stream and a Fischer-Tropsch off-gas.

Suitably, syngas production methods include steam reforming of naturalgas or liquid hydrocarbons and gasification of coal. Methods to convert(gaseous) hydrocarbonaceous feed into syngas include adiabatic oxidativereforming, autothermal reforming and partial oxidation. Preferably,hydrocarbonaceous feed is converted to syngas by partial oxidation atelevated temperature and pressure using an oxygen containing gas.Partial oxidation can take place according to various establishedprocesses. Catalytic as well as non-catalytic processes may be used.These processes include the Shell Gasification Process. A comprehensivesurvey of this process can be found in the Oil and Gas Journal, Sep. 6,1971, pp 86-90.

The H₂/CO ratio of the syngas is suitably between 1.5 and 2.3,preferably between 1.8 and 2.1. The catalysts used for the catalyticconversion of the mixture comprising hydrogen and carbon monoxide intohydrocarbons are known in the art and are usually referred to asFischer-Tropsch catalysts. Preferably, the catalysts for use in theFischer-Tropsch hydrocarbon synthesis process comprises as thecatalytically active component cobalt. The catalytically activecomponent is preferably supported on a porous carrier, e.g. silica ortitania. If desired, the Fischer-Tropsch catalyst may also comprise oneor more metals or metal oxides as promoters. Typically, the catalyticconversion may be effected at a temperature in the range of 150 to 350°C., preferably from 180 to 270° C. Typical total pressures for thecatalytic conversion process are in the range of from 1 to 200 barabsolute, more preferably from 10 to 70 bar absolute.

Generally, the Fischer-Tropsch hydrocarbon product stream is separatedfrom the Fischer-Tropsch off-gas by a gas/liquid separator.

The Fischer-Tropsch off-gas may comprise gaseous hydrocarbons, nitrogen,unconverted methane, unconverted carbon monoxide, carbon dioxide,hydrogen and water. The gaseous hydrocarbons are suitably C₁-C₅hydrocarbons, preferably C₁-C₄ hydrocarbons, more preferably C₁-C₃hydrocarbons. These hydrocarbons, or mixtures thereof, are gaseous attemperatures of 5-30° C. (1 bar), especially at 20° C. (1 bar). Further,oxygenated compounds, e.g. methanol, dimethylether, may be present.

In most cases the Fischer-Tropsh off-gas will contain 10-40 vol %hydrogen, preferably 15-35 vol % hydrogen, 20-65 vol % CO, preferably30-55 vol % CO, 10-50 vol % CO₂, especially 15-45 vol % CO₂, and 10-55vol % N₂, especially 15-50 vol % N₂, calculated on the total volume ofthe gas mixture. Depending on the syngas feed and the Fischer-Tropschconditions the composition of the Fischer-Tropsch off-gas can vary.Obviously, the total volume of the gas mixture is 100 vol %.

The present invention provides a method for recovering carbon monoxideand carbon dioxide from a gas mixture comprising 5-50 vol % methane,10-50 vol % carbon dioxide, 20-65 vol % carbon monoxide, 10-40 vol %hydrogen and 10-55 vol % nitrogen, calculated on the total volume of thegas mixture, said method comprising, in sequence, the following steps:

-   (1) feeding a gas mixture through a column comprising an adsorbent    bed,    -   said gas mixture comprising 5-50 vol % methane, 10-50 vol %        carbon dioxide, 20-65 vol % carbon monoxide, 10-40 vol %        hydrogen and 10-55 vol % nitrogen, calculated on the total        volume of the gas mixture,    -   the adsorbent bed comprising alumina, a carbon molecular sieve,        silicalite, activated carbon, a zeolite, or mixtures thereof,        -   with upon commencement of said feeding, the bed and column            being pre-saturated and pre-pressurized to a pressure in the            range of 10 to 80 bar absolute (bar a), preferably 20 to 70            bar a, with pure hydrogen, or with a mixture of hydrogen and            nitrogen, and        -   discharging effluent from the other end of said bed, and        -   continuing said feeding and said discharging until a carbon            monoxide comprising gas has reached at least 45% of the            length of the bed and has reached at most 80% of the length            of the bed, calculated from the end of the bed at which the            gas mixture is being fed; and-   (2) ceasing the feeding of the gas mixture comprising 5-50 vol %    methane, 10-50 vol % carbon dioxide, 20-65 vol % carbon monoxide,    10-40 vol % hydrogen and 10-55 vol % nitrogen, and reducing the    pressure in the column and the bed by about 5 to 25 bar a; and-   (3) optionally rinsing the column and the adsorbent bed by feeding a    gas comprising at least 70 vol % methane through the column and    adsorbent bed,    -   the column and bed being at a pressure in the range of 5 to 75        bar a, preferably 25 to 65 bar a, more preferably 30 to 55 bar        a, and    -   discharging effluent from the other end of said bed, and    -   continuing said feeding and said discharging until at least 60%        of the carbon monoxide that was present in the bed at the        commencement of this rinsing step is discharged from the other        end of said bed;-   (4) pressurizing the column and adsorbent bed by about 5 to 25 bar a    by feeding a gas comprising at least 70 vol % methane,-   (5) rinsing the column and the adsorbent bed by feeding a gas    comprising at least 70 vol % methane through the column and    adsorbent bed,    -   the column and bed being at a pressure in the range of 30 to 80        bar a, preferably 30 to 70 bar a, and    -   discharging effluent from the other end of said bed, and    -   continuing said feeding and said discharging until an amount        equal to at least 50% of the carbon dioxide that was present at        the commencement of this rinsing step is discharged from the        other end of said bed; and-   (6) ceasing the feeding of a gas comprising at least 70 vol %    methane and reducing the pressure of the column and adsorbent bed to    a pressure in the range of 15 to 25 bar a; and-   (7) further reducing the pressure of the column and adsorbent bed to    a pressure in the range of 1 to 5 bar a; and-   (8) rinsing the column and adsorbent bed by feeding a mixture of    hydrogen and nitrogen through the column and adsorbent bed    -   the column and bed being at a pressure in the range of 1 to 5        bar a, and-   (9) pressurizing the column and adsorbent bed to a pressure in the    range of 15 to 75 bar a, preferably 25 to 65 bar a, more preferably    30 to 55 bar a by feeding a mixture of hydrogen and nitrogen,    whereby said method optionally further comprises the following    steps:-   (A) feeding the effluent of step (1) through a column comprising an    adsorbent bed, the adsorbent bed comprising alumina, a carbon    molecular sieve, silicalite, activated carbon, a zeolite, or    mixtures thereof,    -   with upon commencement of said feeding, the bed and column being        pre-saturated and pre-pressurized to a pressure in the range of        20 to 80 bar absolute (bar a), preferably 30 to 70 bar a, with a        gas comprising more than 95 volume % hydrogen, preferably a gas        comprising 90 to 99.9 volume % hydrogen, and    -   discharging effluent from the other end of said bed, and    -   continuing said feeding and said discharging until a nitrogen        comprising gas has reached at least 45% of the length of the bed        and has reached at most 80% of the length of the bed, calculated        from the end of the bed at which the gas mixture is being fed;        and-   (B) ceasing the feeding of the effluent of step (1), and reducing    the pressure in the column and the bed by about 2 to 5 bar a; and-   (C) further reducing the pressure in the column and the bed by about    2 to 5 bar a; and-   (D) reducing the pressure of the column and adsorbent bed to a    pressure in the range of 1 to 5 bar a; and-   (E) rinsing the column and adsorbent bed by feeding a gas comprising    more than 95 volume % hydrogen, preferably a gas comprising 90 to    99.9 volume % hydrogen, through the column and adsorbent bed    -   the column and bed being at a pressure in the range of 1 to 5        bar a, and-   (F) pressurizing the column and adsorbent bed to a pressure in the    range of 5 to 50 bar a, preferably 10 to 45 bar a, more preferably    20 to 40 bar a by feeding a gas comprising more than 95 volume %    hydrogen, preferably a gas comprising 90 to 99.9 volume % hydrogen,    and-   (G) further pressurizing the column and adsorbent bed to a pressure    in the range of 15 to 75 bar a, preferably 25 to 65 bar a, more    preferably 30 to 55 bar a by feeding a gas comprising more than 95    volume % hydrogen, preferably a gas comprising 90 to 99.9 volume %    hydrogen.

One advantage of the present invention is that the method is suitable toobtain a carbon-monoxide rich stream from Fischer-Tropsch off-gas inoptional step (3). This carbon-monoxide rich stream can be recycled tothe Fischer-Tropsch reactor. A recovery of CO of greater than 50% may beobtained.

The process is also suitable to recover unconverted hydrogen in anefficient way from the Fischer-Tropsch off-gas. Unconverted hydrogen maybe recycled to a Fischer-Tropsch reactor.

Another advantage of the present invention is that the method issuitable to recover a gas comprising methane and carbon dioxide, andoptionally carbon monoxide, from the Fischer-Tropsch off-gas in step(5), whereby this gas can be obtained at high pressure. When the gascomprising carbon dioxide is recovered at a high pressure it can be fedto a gasifier or a reformer, for example an auto thermal reformer,without performing a recompression step.

The process of the present invention makes it possible to convert mostof the carbon atoms of the hydrocarbonaceous feed stock to the desiredC₅+ hydrocarbons.

In a preferred embodiment of the present invention repeated cycles ofsteps (1) to (9) are performed. In a preferred embodiment step (3) isperformed. Effluent from one step can be used as feeding gas in anotherstep; this is especially advantageous when repeated cycles of steps (1)to (9) are performed.

The method may be performed using a single column comprising anadsorbent bed. Preferably several columns that comprise an adsorbent bedare used. When using more than one column, the columns are preferablyconnected in parallel. Preferably the repeated cycles of steps (1) to(9) are performed over each column. In a preferred embodiment at leastone column is subjected to one step of the cycle while another column issubjected to another step of the cycle. The product of one column can beused in another column, for example for purge, pressurization or rinse.In one embodiment at least two columns comprising an adsorbent bed,preferably at least six columns comprising an adsorbent bed, aresubjected to repeated cycles of steps (1) to (9). Preferably at most 21,more preferably at most 16, columns comprising an adsorbent bed aresubjected to repeated cycles of steps (1) to (9).

The gas mixture comprising 5-50 vol % methane, 10-50 vol % carbondioxide, 20-65 vol % carbon monoxide, 10-40 vol % hydrogen and 10-55 vol% nitrogen, calculated on the total volume of the gas mixture,preferably is a gaseous product from a Fischer-Tropsch reaction. In thatcase it may be referred to as Fischer-Tropsch off-gas. Fischer-Tropschoff-gas is typically produced by a Fischer-Tropsch hydrocarbon synthesisprocess comprising the steps of:

i) conversion of a (gaseous) hydrocarbonaceous feed to obtain synthesisgas (syngas);ii) catalytic conversion of the synthesis gas obtained in step i) usinga Fischer-Tropsch catalyst into a Fischer-Tropsch product; andiii) separating the Fischer-Tropsch product of step ii) into at leastone hydrocarbon product stream and a Fischer-Tropsch off-gas.The off-gas obtained in step iii) may comprise gaseous hydrocarbons,nitrogen, unconverted methane, unconverted carbon monoxide, carbondioxide, hydrogen and water. The gaseous hydrocarbons are suitably C₁-C₅hydrocarbons, preferably C₁-C₄ hydrocarbons, more preferably C₁-C₃hydrocarbons. These hydrocarbons, or mixtures thereof, are gaseous attemperatures of 5-30° C. (1 bar), especially at 20° C. (1 bar). Further,oxygenated compounds, e.g. methanol, dimethylether, may be present.

The gas mixture comprising 5-50 vol % methane, 10-50 vol % carbondioxide, 20-65 vol % carbon monoxide, 10-40 vol % hydrogen and 10-55 vol% nitrogen, calculated on the total volume of the gas mixture,preferably comprises less than 10 volume %, more preferably less than 5volume %, of hydrocarbons having 6 or more carbon atoms. More preferablythe gas mixture comprises less than 10 volume %, more preferably lessthan 5 volume %, of hydrocarbons having 5 or more carbon atoms. Evenmore preferably the gas mixture comprises less than 10 volume %, morepreferably less than 5 volume %, of hydrocarbons having 4 or more carbonatoms. In one embodiment hydrocarbons having 3 or more carbon atoms areremoved from a Fischer-Tropsch off-gas, for example using a scrubber,before it is subjected to the method of the present invention. The gasmixture comprising 5-50 vol % methane, 10-50 vol % carbon dioxide, 20-65vol % carbon monoxide, 10-40 vol % hydrogen and 10-55 vol % nitrogen,calculated on the total volume of the gas mixture, may comprise ethane.In a preferred embodiment the amount of ethane is less than 5 wt %, morepreferably less than 2 wt %, and even more preferably less than 1 wt %,calculated on the total weight of the gas mixture.

The feed rate to step (1) may, for example be about 5000 to 20000kmoles/hr.

For steps (1) to (9), the column comprising an adsorbent bed may be madeof metal, preferably stainless steel. The adsorbent bed for steps (1) to(9) comprises alumina, a carbon molecular sieve, silicalite, activatedcarbon, a zeolite, or mixtures thereof. Preferably the adsorbent bedcomprises activated carbon and/or zeolite. In another preferredembodiment the adsorbent bed comprises activated carbon and/orsilicalite. When alumina is used, it is preferably combined with acarbon molecular sieve, activated carbon silicalite and/or zeolite. Ifzeolite is used, it may for example be ZSM-5A and/or ZSM-13 X. Ifsilicalite is used, preferably a silicalite with a high silica toalumina molar ratio (SAR) is used. Activated carbon, silicalite andzeolite hardly adsorb nitrogen and hydrogen, but do adsorb carbonmonoxide, methane, and carbon dioxide. Activated carbon, silicalite andzeolite are preferential adsorbents for methane and carbon dioxide ascompared to carbon monoxide.

The absorbent bed and column are already pre-saturated andpre-pressurized with hydrogen or with a mixture of hydrogen and nitrogenupon commencement of feeding the gas mixture comprising 5-50 vol %methane, 10-50 vol % carbon dioxide, 20-65 vol % carbon monoxide, 10-40vol % hydrogen and 10-55 vol % nitrogen in step (1). The bed and columnmay be saturated and pressurized with pure hydrogen. Pure hydrogencomprises more than 90 volume %, preferably more than 95 vol %, and morepreferably more than 99.9 vol % of hydrogen. The bed and column may besaturated and pressurized with a mixture of hydrogen and nitrogen. Themixture of hydrogen and nitrogen preferably comprises hydrogen in arange of between 60 to 95 vol %, and nitrogen in a range of between 5 to40 vol %. The mixture of hydrogen and nitrogen preferably comprises lessthan 10 vol %, more preferably less than 5 vol %, even more preferablyless than 1 vol % of gasses other than hydrogen and nitrogen. Forexample, the bed and column may be saturated and pressurized with aproduct hydrogen and nitrogen comprising gas from step (1) of an earliercycle.

In step (1), the gas mixture is fed to one end of the adsorbent bed, andeffluent is discharged from the other end of the adsorbent bed. Theadsorbent bed will adsorb methane, carbon dioxide, nitrogen and carbonmonoxide. The effluent will mainly comprise hydrogen and nitrogen. Apart of this effluent can be used to pressurize a column and absorbentbed at the start of a cycle or in step (9). Another part of thiseffluent can be used in rinsing step (8). Another part can be sent asfeed to a Fischer-Tropsch reaction, even though it comprises nitrogen.The pressure of the effluent gas will be about the same as the pressurein the column and the adsorbent bed and will thus be in the range of 10to 80 bar absolute (bar a), preferably 20 to 70 bar a.

In step (1), the feeding of the gas mixture and the discharging of theeffluent are continued until a carbon monoxide comprising gas hasreached at least 45% of the length of the bed, preferably at least 50%,more preferably at least 60%, and has reached at most 80% of the lengthof the bed, preferably at most 70%, calculated from the end of the bedat which the gas mixture is being fed. In a preferred embodiment, thefeeding and discharging is ceased when the adsorption capacity of theadsorbent bed towards carbon monoxide is reduced by 50% to 90%,preferably 50% to 80%, more preferably 60% to 70% from its adsorptioncapacity towards carbon monoxide when commencing the feeding of the gasmixture.

The progress of a carbon monoxide comprising gas through the bed can bemonitored. This may, for example, be performed by analyzing gas samplesof the effluent and/or gas samples from the column and adsorbent bed.The progress of a carbon monoxide comprising gas through the bed mayadditionally or alternatively be monitored by determining thetemperature along the length of the bed, e.g. by using thermocouplesplaced along the length of the bed. At the front of the carbon monoxidecomprising gas the temperature is increased as compared to the part ofthe bed that has not yet been reached by the carbon monoxide comprisinggas. At the front of the carbon monoxide comprising gas the temperatureis also increased as compared to the part of the bed where the carbonmonoxide has already been adsorbed in the adsorbent bed.

In step (2), ceasing the feeding of the gas mixture, which comprises5-50 vol % methane, 10-50 vol % carbon dioxide, 20-65 vol % carbonmonoxide, 10-40 vol % hydrogen and 10-55 vol % nitrogen, is performed bystopping the flow of gas to the column comprising an adsorbent bed. Whenthe feeding and discharging is stopped, some hydrogen and nitrogen willremain in the column containing an adsorbent bed. Upon ceasing thefeeding of the gas mixture, the pressure in the column and adsorbent bedis reduced by about 5 to 10 bar a. The pressure reduction in the columnand adsorbent bed suffices to let most of the effluent, which compriseshydrogen and nitrogen, leave. The effluent leaves the column and bed atthe same end from which effluent was discharged in step (1).

In optional step (3), a gas comprising at least 70 vol % methane is fedto the column comprising an adsorbent bed. In optional step (3)preferably a gas comprising at least 90 vol % methane is fed to thecolumn comprising an adsorbent bed. In optional step (3), the gas is fedto the same end of the bed at which a gas mixture, for exampleFischer-Tropsch off-gas, was fed in step (1).

The gas comprising at least 70 vol % methane, preferably at least 90 vol% methane, may, for example, be pure methane, a mixture of methane andcarbon dioxide, or treated natural gas. Treated natural gas is naturalgas from which contaminants like water and sulfur have been removed. Thegas comprising at least 70 vol % methane, preferably at least 90 vol %methane, may, for example, have the same composition as thehydrocarbonaceous feed that is converted into syngas for theFischer-Tropsch reaction.

The gas comprising at least 70 vol % methane, preferably at least 90 vol% methane, may be or may comprise the effluent of step (6). Hence, thebed and column may be fed with a product methane comprising gas fromstep (6) of an earlier cycle. In that case the product methanecomprising gas from step (6) of an earlier cycle may be pressurizedbefore it is used as feeding gas in step (3), but this may not benecessary as the product gas from step (6) may be at a sufficiently highpressure when it leaves the column at step (6).

In step (3), effluent is discharged from the other end of the adsorbentbed. The adsorbent bed will adsorb even more methane while a methanecomprising gas is fed. The effluent will comprise carbon monoxide.Preferably the effluent comprises at least 50 vol %, preferably at least80 vol %, more preferably at least 90 vol %, even more preferably atleast 95 vol %, still more preferably at least 99 vol % carbon monoxide,calculated on the total volume of the effluent. The pressure of theeffluent gas will be about the same as the pressure in the column andthe adsorbent bed and will thus be in the range of 5 to 75 bar absolute(bar a), preferably 25 to 65 bar a, more preferably 30 to 55 bar a. Thecarbon monoxide product stream can be used as feed for a Fischer-Tropschreaction. It can, for example, be a recycle stream in a Fischer-Tropschprocess. This is very advantageous as it makes it possible to convertmost of the carbon atoms of the hydrocarbonaceous feed stock to thedesired C₅+ hydrocarbons.

In one embodiment of the method of the present invention, an optionallyscrubbed Fischer-Tropsch off-gas is used in step (1), and at least apart of the effluent of step (3) is sent as a recycle stream to theFischer-Tropsch reactor that produced the off-gas.

In another embodiment, an optionally scrubbed Fischer-Tropsch off-gasfrom a first Fischer-Tropsch reactor is used in step (1), and at least apart of the effluent of step (3) is sent as a feed stream to a secondFischer-Tropsch reactor.

In a further embodiment at least a part of the effluent of step (3) issent as a recycle stream to the Fischer-Tropsch reactor that producedthe off-gas, and at least a part of the effluent of step (3) is sent asa feed stream to a second Fischer-Tropsch reactor.

In optional step (3), the feeding of a methane comprising gas, and thedischarging of the effluent are continued until at least 60%, preferablyat least 70%, more preferably at least 80%, even more preferably atleast 90%, still more preferably at least 95% of the carbon monoxidethat was present in the adsorbent bed at the commencement of thisrinsing step is discharged from the other end of the bed. The methane inthe feed will replace carbon monoxide in the adsorbent bed. In oneembodiment, methane is fed in step (3) and the feeding and dischargingis ceased when a breakthrough of methane is imminent. In thisembodiment, methane is thus fed until the adsorption capacity of theadsorbent bed towards methane is nil or almost nothing.

The progress of a methane comprising gas through the bed can bemonitored, e.g. by analyzing gas samples of the effluent and/or gassamples from the column and adsorbent bed. The progress of a methanecomprising gas through the bed may additionally or alternatively bemonitored by determining the temperature along the length of the bed,e.g. by using thermocouples placed along the length of the bed. At thefront of the methane comprising gas the temperature is increased ascompared to the part of the bed that has not yet been reached by themethane comprising gas. At the front of the methane comprising gas thetemperature is also increased as compared to the part of the bed wherethe methane has already replaced carbon monoxide in the adsorbent bed.

In step (4), a gas comprising at least 70 vol % methane is fed to thecolumn comprising an adsorbent bed. In step (4) preferably a gascomprising at least 90 vol % methane is fed to the column comprising anadsorbent bed. In step (4), the gas is fed to the same end of the bed atwhich a gas mixture was fed in step (1). The gas comprising at least 70vol % methane, preferably at least 90 vol % methane, may, for example,be pure methane, a mixture of methane and carbon dioxide, or treatednatural gas.

In step (5), a gas comprising at least 70 vol % methane is fed to thecolumn comprising an adsorbent bed. In step (5) preferably a gascomprising at least 90 vol % methane is fed to the column comprising anadsorbent bed. In step (5), the gas is fed to the same end of the bed atwhich a gas mixture was fed in step (1). The gas comprising at least 70vol % methane, preferably at least 90 vol % methane, may, for example,be pure methane, a mixture of methane and carbon dioxide, or treatednatural gas.

In step (5), effluent is discharged from the other end of the adsorbentbed. The effluent will comprise methane and carbon dioxide. The pressureof the effluent gas will be about the same as the pressure in the columnand the adsorbent bed and will thus be in the range of 30 to 80 barabsolute (bar a), preferably 30 to 70 bar a. A product stream comprisingmethane and carbon dioxide is obtained. In case optional step (3) is notperformed, the product stream will comprise carbon monoxide, methane andcarbon dioxide.

The gas comprising methane and carbon dioxide, and optionally carbonmonoxide, discharged in step (5), can be obtained at high pressure. Whenthe gas comprising methane and carbon dioxide is recovered at a highpressure it can be fed to a gasifier or a reformer, for example an autothermal reformer, without performing a pressurization step. The processof the present invention makes it possible to convert most of the carbonatoms of the hydrocarbonaceous feed stock to the desired C₅+hydrocarbons.

The effluent discharged in step (5) may be fed to a gasifier. Thegasification may be carried out by partially oxidating, for exampleaccording to the shell gasification process (SGP) by partial oxidationusing pure oxygen. The partial oxidation using pure oxygen may beoperated at 1100 to 1700° C. Preferably the partial oxidation using pureoxygen is operated at 1300 to 1500° C. and pressures up to 70 bar. Thegasification may be carried out by partial oxidation as described inW09603345A1 using a co-annular burner using 99.5% pure oxygen andoptionally carbon dioxide as moderator gas and in the absence of acatalyst. A further example is described in W02008006787A2. In theprocess of W02008006787A2 partial oxidation on a methane comprising feedis performed using a multi-orifice burner provided with an arrangementof separate passages, wherein the gaseous hydrocarbon having at elevatedtemperature flows through a passage of the burner, an oxidiser gas flowsthrough a separate passage of the burner and wherein the passage forgaseous hydrocarbon feed and the passage for oxidiser gas are separatedby a passage through which a secondary gas flows, wherein the secondarygas comprises hydrogen, carbon monoxide and/or a hydrocarbon.

The effluent discharged in step (5) may be fed to a reformer. Thereforming may be carried out using a reforming process, preferably usinga steam reforming process. More preferably a steam methane reformingprocess (SMR), an adiabatic steam reforming process (ASR), a fired steamreforming process, or an auto thermal steam reforming process (ATR) isused. Most preferably an auto thermal steam reforming process (ATR) isused. Even more preferably an ATR process is used in which gas heatedreforming (GHR) is incorporated. The ATR and the GHR can be linked indifferent ways; the configuration in which feed gas passes through theGHR and ATR in series is preferred. Hence, the effluent discharged instep (5) most preferably is fed to an ATR, even more preferably to a GHRand ATR which are placed in series.

In step (5), the feeding of a gas comprising at least 70 vol % methaneand the discharging of the effluent are continued until an amount equalto at least 50%, preferably at least 60%, more preferably 50 to 80%,even more preferably 60 to 80%, of the carbon dioxide that was presentat the commencement of this rinsing step is discharged from the otherend of said bed. The aim of this is to recover the carbon dioxide thatwas present in the gas fed to step (1), which preferably isFischer-Tropsch off-gas.

The velocity of the feed may be relatively low. The feed rate to step(5) may, for example be about 5000 to 10000 kmoles/hr.

One way to control the amount of carbon dioxide in the effluent in step(5) is to determine the amount of carbon dioxide in the gas fed to step(1) and to determine the amount of carbon dioxide that is discharged instep (5). The feeding and said discharging are continued in step (5) isuntil an amount equal to at least 50% of the carbon dioxide that waspresent at the commencement of this rinsing step is discharged from theother end of said bed.

Another way to control the amount of carbon dioxide in the effluent ofstep (5) is to control the feed ratios to steps (1) and (5). When thecomposition and the feed rate of step (1) are known, and the compositionand the feed rate of step (5) are known, one can calculate the amount ofcarbon dioxide that is recovered in step (5) from the gas fed to step(1). Preferably the ratio between the flow rate of the gas fed to step(1) and the flow rate of the gas fed to step (5) is in the range ofbetween 0.1 to 2, preferably 0.5 to 2, more preferably 0.8 to 1.2. Thefeed rate to step (1) may, for example be about 5000 to 20000 kmoles/hr.

This way of controlling the amount of carbon dioxide in the effluent ofstep (5) is especially suitable when repeated cycles of steps (1) to (9)are performed. This way of controlling the amount of carbon dioxide inthe effluent of step (5) is even more suitable when repeated cycles ofsteps (1) to (9) are performed in several columns which are connected inparallel. When repeated cycles of steps (1) to (9) are performed, percycle the feeding and said discharging are continued in step (5) untilan amount equal to at least 50% of the carbon dioxide that was presentat the commencement of this rinsing step is discharged from the otherend of said bed.

In step (6), the feeding of a gas comprising at least 70 vol % methaneis ceased. The pressure of the column and adsorbent bed is reduced to apressure in the range of 15 to 25 bar a. During step (6) a mixture ofmethane and carbon dioxide leaves the column and bed at the same end atwhich in step (5) a gas comprising at least 70 vol % methane was fed tothe column and bed. All or a part of this effluent may be sent to a fuelgas pool or to an SMR. Preferably a part of this effluent is used asfeed for step (3).

In step (7), the pressure of the column and adsorbent bed is furtherreduced to a pressure in the range of 1 to 5 bar a. During step (7) amixture of methane and carbon dioxide leaves the column and bed at thesame end at which in step (5) a gas comprising at least 70 vol % methanewas fed to the column and bed. This effluent may be sent to a fuel gaspool.

In step (8), the column and adsorbent bed are rinsed by feeding amixture of hydrogen and nitrogen through the column and adsorbent bed.The mixture of hydrogen and nitrogen is fed to the same end of thecolumn and bed from which effluent was discharged in step (1). Duringstep (8) residual methane and carbon dioxide leaves the column and bedat the same end at which in step (1) a gas mixture was fed to the columnand bed.

In a preferred embodiment, the mixture of hydrogen and nitrogen used instep (8) is a part of the effluent from step (2), and may optionallyalso comprise a part of the effluent from step (1).

In a preferred embodiment, the mixture of hydrogen and nitrogen used instep (8) is a part of the effluent from step (D) and/or step (E).

In a preferred embodiment, the mixture of hydrogen and nitrogen used instep (8) is a part of the effluent from step (2) and a part of theeffluent from step (D) and/or step (E). It may optionally also comprisea part of the effluent from step (1).

The gas mixture fed to the column and bed rinses the bed from methaneand carbon dioxide. The pressure of the effluent gas will be about thesame as the pressure in the column and the adsorbent bed and will thusbe in the range of 1 to 5 bar a. The effluent can be sent to a fuelpool.

In a preferred embodiment, the column and adsorbent bed are rinsed instep (8) by feeding a gas comprising at least 95 vol % nitrogen,preferably at least 99 vol % nitrogen, through the column and adsorbentbed, followed by feeding a mixture of hydrogen and nitrogen through thecolumn and adsorbent bed, whereby the column and bed are at a pressurein the range of 1 to 5 bar a. The gas comprising at least 95 vol %nitrogen, preferably at least 99 vol % nitrogen, may be the product ofan air separation unit (ASU). The mixture of hydrogen and nitrogenpreferably is a part of the effluent from step (2) and/or steps (D) and(E), and may optionally also comprise a part of the effluent from step(1). The gas comprising at least 95 vol % nitrogen, and in a subsequentstep the mixture of hydrogen and nitrogen, are fed to the same end ofthe column and bed from which effluent was discharged in step (1). Whenthe gas comprising at least 95 vol % nitrogen is fed to the column andbed, a part of the residual methane and carbon dioxide leaves the columnand bed at the same end at which in step (1) a gas mixture was fed tothe column and bed. This effluent can be sent to a fuel pool.

In step (9) the column and adsorbent bed are pressurized to a pressurein the range of 15 to 75 bar a, preferably 25 to 65 bar a, morepreferably 30 to 55 bar a by feeding a mixture of hydrogen and nitrogen.The mixture used may comprise a part of the product hydrogen andnitrogen from step (1), and may optionally also comprise a part of theproduct hydrogen and nitrogen from step (2). The mixture of hydrogen andnitrogen preferably comprises hydrogen in a range of between 60 to 95vol %, and nitrogen in a range of between 5 to 40 vol %. The mixture ofhydrogen and nitrogen preferably comprises less than 10 vol %, morepreferably less than 5 vol %, even more preferably less than 1 vol % ofgases other than hydrogen and nitrogen.

In a preferred embodiment, steps (A) to (G) are performed. In thispreferred embodiment, a gas comprising at least 80 vol % hydrogen,calculated on the total volume of this hydrogen comprising gas, isproduced. The gas preferably comprising at least 80 vol % hydrogen isobtained in steps (A), (B), and (C).

The gas comprising at least 80 vol % hydrogen may be recycled to aFischer-Tropsch reactor.

As described above, steps (1) to (9) may be performed using a singlecolumn comprising an adsorbent bed. As described above, preferablyrepeated cycles of steps (1) to (9) are performed. Preferably at leasttwo, preferably at least six, columns that comprise an adsorbent bed areused for steps (1) to (9). Preferably the columns for steps (1) to (9)are connected in parallel.

In a preferred embodiment of the present invention repeated cycles ofsteps (A) to (G) are performed. Effluent from one step can be used asfeeding gas in another step; this is especially advantageous whenrepeated cycles of steps (A) to (G) are performed.

Steps (A) to (G) may be performed using a single column comprising anadsorbent bed. Preferably several columns that comprise an adsorbent bedare used. When using more than one column, the columns are preferablyconnected in parallel. Preferably the repeated cycles of steps (A) to(G) are performed over each column. In a preferred embodiment at leastone column is subjected to one step of the cycle while another column issubjected to another step of the cycle. The product of one column can beused in another column, for example for purge, pressurization or rinse.In one embodiment at least two columns comprising an adsorbent bed,preferably at least five columns comprising an adsorbent bed, aresubjected to repeated cycles of steps (A) to (G). Preferably at most 20,more preferably at most 15, columns comprising an adsorbent bed aresubjected to repeated cycles of steps (A) to (G).

Preferably at least two, preferably at least six, columns that comprisean adsorbent bed are used for steps (1) to (9) and at least two,preferably at least five, columns that comprise an adsorbent bed areused for steps (A) to (G).

Preferably one column or one set of columns is used for steps (1) to (9)and another column or another set of columns is used for steps (A) to(G).

Preferably the columns for steps (1) to (9) are connected in parallel,and the columns for steps (A) to (G) are connected in parallel.

In case steps (1) to (9) and steps (A) to (G) are performed, preferablyuse is made of two different pressure swing adsorption units (PSAunits); one PSA for steps (1) to (9), and one PSA for steps (A) to (G).

The feed gas for step (A) is at least a part of the effluent of step(1).

The feed gas for step (A) preferably comprises 10 to 40 volume %nitrogen and 60 to 90 volume % hydrogen. More preferably the feed gasfor step (A) comprises 15 to 35 volume % nitrogen and 65 to 85 volume %hydrogen.

For steps (A) to (G), the column comprising an adsorbent bed may be madeof metal, preferably stainless steel. The adsorbent bed for steps (A) to(G) comprises alumina, a carbon molecular sieve, silicalite, activatedcarbon, a zeolite, or mixtures thereof. Preferably the adsorbent bedcomprises a zeolite. When alumina is used, it is combined with azeolite. The zeolite may for example be ZSM-5A and/or ZSM-13 X and/or aLiX zeolite. Zeolite is a preferential adsorber for nitrogen as comparedto hydrogen.

The absorbent bed and column are already pre-saturated andpre-pressurized with hydrogen upon commencement of feeding the effluentof step (1) in step (A). The bed and column may be saturated andpressurized with pure hydrogen. Pure hydrogen comprises more than 90volume %, preferably more than 95 vol %, and more preferably more than99.9 vol % of hydrogen. The bed and column may be saturated andpressurized with a product hydrogen comprising gas from step (A) of anearlier cycle.

In step (A), effluent is discharged from the other end of the adsorbentbed. The adsorbent bed will adsorb nitrogen. The effluent will mainlycomprise hydrogen. A part of this effluent can be used to pressurize acolumn and absorbent bed at the start of a cycle or in step (G). Anotherpart can be sent as feed to a Fischer-Tropsch reaction. The pressure ofthe effluent gas will be about the same as the pressure in the columnand the adsorbent bed and will thus be in the range of 20 to 80 barabsolute (bar a), preferably 30 to 70 bar a.

In step (A), the feeding of the gas mixture and the discharging of theeffluent are continued until a front of nitrogen comprising gas hasreached at least 45% of the length of the bed, preferably at least 50%,more preferably at least 60%, and has reached at most 80% of the lengthof the bed, preferably at most 70%, calculated from the end of the bedat which the gas mixture is being fed. In a preferred embodiment, thefeeding and discharging is ceased when the adsorption capacity of theadsorbent bed towards nitrogen is reduced by 50% to 80%, preferably 60%to 70% from its adsorption capacity towards nitrogen when commencing thefeeding of the gas mixture. The progress of a nitrogen comprising gasthrough the bed can be monitored, e.g. by analyzing gas samples of theeffluent and/or gas samples from the column and adsorbent bed. Theprogress of a nitrogen comprising gas through the bed may additionallyor alternatively be monitored by determining the temperature along thelength of the bed, e.g. by using thermocouples placed along the lengthof the bed. At the front of the nitrogen comprising gas the temperatureis increased as compared to the part of the bed that has not yet beenreached by the nitrogen comprising gas. At the front of the nitrogencomprising gas the temperature is also increased as compared to the partof the bed where nitrogen has already been adsorbed.

In step (B), ceasing the feeding of the gas mixture is performed bystopping the flow of gas to the column comprising an adsorbent bed. Whenthe feeding and discharging is stopped, some hydrogen and nitrogen willremain in the column containing an adsorbent bed. Upon ceasing thefeeding of the gas mixture, the pressure in the column and adsorbent bedis preferably reduced by in total about 5 to 10 bar a in steps (B) and(C). Most preferably steps (B) and (C) are performed as separate steps.It is also possible to combine steps (B) and (C) and reduce the pressurein the column and absorbent bed by in total about 5 to 10 bar a in asingle step. When steps (B) and (C) are preformed separately, thepressure in the column and bed is reduced in step (B) by about 2 to 5bar a, and is further reduced in step (C) by another 2 to 5 bar a. Thepressure reduction in the column and adsorbent bed suffices to let mostof the effluent, which comprises hydrogen, leave. During steps (B) and(C) the effluent leaves the column and bed at the same end from whicheffluent was discharged in step (A).

In step (D), the pressure of the column and adsorbent bed is reduced toa pressure in the range of 1 to 5 bar a. During step (D) hydrogen andnitrogen leave the column and bed at the same end at which in step (A)effluent of step (1) was fed to the column and bed. The effluent can besent to a fuel pool. Additionally or alternatively at least a part ofthe effluent of step (D) can be used as a feeding gas in step (8).

During step (D) almost all hydrogen and nitrogen leave the column andbed.

In step (E), the column and adsorbent bed are rinsed by feeding hydrogenthrough the column and adsorbent bed. The hydrogen is fed to the sameend of the column and bed from which effluent was discharged in step(A). During step (E) residual nitrogen leaves the column and bed at thesame end at which in step (A) an effluent from step (1) was fed to thecolumn and bed.

In a preferred embodiment, the hydrogen used in step (E) is a part ofthe effluent from step (C), and may optionally also comprise a part ofthe effluent from step (B) and/or (A).

Optionally, in step (E) the column is first rinsed with effluent fromstep (C) before it is rinsed by feeding a gas comprising more than 95volume % hydrogen, preferably a gas comprising 90 to 99.9 volume %hydrogen, through the column and adsorbent bed.

The hydrogen fed to the column and bed in step (E) rinses the bed fromnitrogen. The pressure of the effluent gas will be about the same as thepressure in the column and the adsorbent bed and will thus be in therange of 1 to 5 bar a. The effluent can be sent to a fuel pool.Additionally or alternatively at least a part of the effluent of step(E) can be used as a feeding gas in step (8).

In steps (F) and (G) the column and adsorbent bed are pressurized to apressure in the range of 15 to 75 bar a, preferably 25 to 65 bar a, morepreferably 30 to 55 bar a by feeding hydrogen. In step (F), the hydrogenused may be or may comprise a part of the product hydrogen from step(B), and may optionally also comprise a part of the product hydrogenfrom step (A). In step (G), the hydrogen preferably is a part of theproduct hydrogen from step (A). Most preferably steps (F) and (G) areperformed as separate steps. When steps (F) and (G) are preformedseparately, the column and bed are pressurized in step (F) to a pressurein the range of 5 to 50 bar a, preferably 10 to 45 bar a, morepreferably 20 to 40 bar a, and is further pressurized in step (G) to apressure in the range of 15 to 75 bar a, preferably 25 to 65 bar a, morepreferably 30 to 55 bar a. It is also possible to combine steps (F) and(G) and pressurize the column and absorbent bed to a pressure in therange of 15 to 75 bar a, preferably 25 to 65 bar a, more preferably 30to 55 bar a by feeding hydrogen in a single step using hydrogen fromstep (A) and optionally also hydrogen from step (B).

The hydrogen fed to the column in steps (E), (F) and (G) preferably ispure hydrogen. The hydrogen fed to the column in steps (E), (F) and (G)preferably is a gas comprising more than 95 volume % hydrogen, morepreferably a gas comprising 90 to 99.9 volume % hydrogen. Rinsing step(E) may be performed with product hydrogen comprising gas of steps (A),(B) and/or (C).

In a another preferred embodiment, the invention provides a method forrecovering carbon monoxide and carbon dioxide from a gas mixturecomprising 5-50 vol % methane, 10-50 vol % carbon dioxide, 20-65 vol %carbon monoxide, 10-40 vol % hydrogen and 10-55 vol % nitrogen,calculated on the total volume of the gas mixture, said methodcomprising, in sequence, the following steps:

-   (1) feeding a gas mixture through a column comprising an adsorbent    bed,    -   said gas mixture comprising 5-50 vol % methane, 10-50 vol %        carbon dioxide, 20-65 vol % carbon monoxide, 10-40 vol %        hydrogen and 10-55 vol % nitrogen, calculated on the total        volume of the gas mixture,    -   the adsorbent bed comprising alumina, a carbon molecular sieve,        silicalite, activated carbon, a zeolite, or mixtures thereof,        -   with upon commencement of said feeding, the bed and column            being pre-saturated and pre-pressurized to a pressure in the            range of 10 to 80 bar absolute (bar a), preferably 20 to 70            bar a, with pure hydrogen, or with a mixture of hydrogen and            nitrogen, and        -   discharging effluent from the other end of said bed, and        -   continuing said feeding and said discharging until a carbon            monoxide comprising gas has reached at least 45% of the            length of the bed and has reached at most 80% of the length            of the bed, calculated from the end of the bed at which the            gas mixture is being fed; and-   (2) ceasing the feeding of the gas mixture comprising 5-50 vol %    methane, 10-50 vol % carbon dioxide, 20-65 vol % carbon monoxide,    10-40 vol % hydrogen and 10-55 vol % nitrogen, and reducing the    pressure in the column and the bed by about 5 to 25 bar a; and-   (3) optionally rinsing the column and the adsorbent bed by feeding a    gas comprising at least 70 vol % methane through the column and    adsorbent bed,    -   the column and bed being at a pressure in the range of 5 to 75        bar a, preferably 25 to 65 bar a, more preferably 30 to 55 bar        a, and    -   discharging effluent from the other end of said bed, and    -   continuing said feeding and said discharging until at least 60%        of the carbon monoxide that was present in the bed at the        commencement of this rinsing step is discharged from the other        end of said bed;-   (4) pressurizing the column and adsorbent bed by about 5 to 25 bar a    by feeding a gas comprising at least 70 vol % methane,-   (5) rinsing the column and the adsorbent bed by feeding a gas    comprising at least 70 vol % methane through the column and    adsorbent bed,    -   the column and bed being at a pressure in the range of 30 to 80        bar a, preferably 30 to 70 bar a, and    -   discharging effluent from the other end of said bed, and    -   continuing said feeding and said discharging until an amount        equal to at least 50% of the carbon dioxide that was present at        the commencement of this rinsing step is discharged from the        other end of said bed; and-   (6) ceasing the feeding of a gas comprising at least 70 vol %    methane and reducing the pressure of the column and adsorbent bed to    a pressure in the range of 15 to 25 bar a; and-   (7) further reducing the pressure of the column and adsorbent bed to    a pressure in the range of 1 to 5 bar a; and-   (8) rinsing the column and adsorbent bed by feeding a mixture of    hydrogen and nitrogen through the column and adsorbent bed    -   the column and bed being at a pressure in the range of 1 to 5        bar a, and-   (9) pressurizing the column and adsorbent bed to a pressure in the    range of 15 to 75 bar a, preferably 25 to 65 bar a, more preferably    30 to 55 bar a by feeding a mixture of hydrogen and nitrogen,    said method further comprising, in sequence, the following steps:-   (I) sending at least a part of the discharged effluent of step (1)    as feed to a membrane unit,-   (II) optionally sending at least a part of the discharged effluent    of step (3) as sweep gas to the membrane unit,-   (III) optionally sending at least a part of the retentate of the    membrane unit as feed to step (8),-   (IV) sending at least a part of the permeate of the membrane unit as    feed to a Fischer-Tropsch reaction.

In this preferred embodiment of the present invention a gas streamcomprising hydrogen and carbon monoxide is produced which may be used asfeed for a Fischer-Tropsch reaction. The produced gas comprising atleast 80% carbon monoxide, which is effluent of step (3), may be partlyor completely used to produce the gas stream comprising hydrogen andcarbon monoxide.

In a preferred embodiment steps (II) and (III) are performed.

Steps (I) to (IV) and steps (A) to (G) as described above preferably arenot combined. The process of the present invention preferably has eitheradditional steps (A) to (G) or additional steps (I) to (IV).

The membrane unit that can be used can comprise a polymeric membrane ora ceramic membrane, preferably a polymeric membrane, most preferably apolyimide membrane. Such membranes are commercially available.

Discharged effluent of step (1) comprises nitrogen and hydrogen and issent as feed to a membrane unit. Also a sweep gas is sent to themembrane unit. The sweep gas preferably comprises carbon monoxide andmay be discharged effluent of step (3).

In the membrane unit, feed comprising nitrogen and hydrogen is sent toone side of the membrane and sweep gas comprising carbon monoxide issent to the other side of the membrane. The feed comprising nitrogen andhydrogen which is sent as feed to one side of the membrane preferably isat a higher pressure as compared to the sweep gas comprising carbonmonoxide which is sent to the other side of the membrane. The feed may,for example, be at a pressure in the range of from 30 to 70 bar a.

In the membrane unit, hydrogen flows through the membrane. Preferablynitrogen and carbon monoxide do not or hardly flow through the membrane.Hydrogen present in the feed to the membrane unit becomes part of thepermeate. The permeate of the membrane unit comprises carbon monoxideand hydrogen. Permeate may be sent as feed to a Fischer-Tropschreaction. The retentate of the membrane unit comprises nitrogen.Retentate may be sent as feed to step (8).

Some embodiments of the method according to the invention will beillustrated below with reference to the attached figures. It is notedthat the present invention should not be considered limited thereto orthereby.

FIG. 1 illustrates an overview of the process steps of a preferredmethod according to the invention. One column comprising an adsorbentbed is depicted nine times; each time it shows a step of steps (1) to(9) according to the invention. Another column comprising an adsorbentbed is depicted seven times; each time it shows a step of steps (A) to(G) according to the invention. In step (1) Fischer-Tropsch off-gas isfed to the column and bed and a mixture of hydrogen and nitrogen isdischarged. During step (1) the pressure in the column and bed is high.In step (2) the pressure is reduced by 5 to 10 bar a, and a remainder ofhydrogen and nitrogen is discharged. In step (3) the column and bed arerinsed by feeding them with treated natural gas and/or the effluent ofstep (6) of an earlier cycle and discharging carbon monoxide. The carbonmonoxide stream can be sent as feed to a Fischer-Tropsch reaction. Instep (4) the column and bed are pressurized by feeding with treatednatural gas at high pressure. In step (5) the column and bed are rinsedby feeding them with treated natural gas and discharging a mixture ofmethane and carbon dioxide. The mixture of methane and carbon dioxide isobtained as a product stream and is at a high pressure. This product maybe fed to a gasifier or a reformer, for example an auto thermalreformer, without performing a pressurization step. In step (6) thepressure in the column and bed is reduced to 15 to 25 bar a, and amixture of methane and carbon dioxide is discharged. A part of thismixture may be used as feed for step (3). In step (7) the pressure inthe column and bed is further reduced to 1 to 5 bar a, and a mixture ofmethane and carbon dioxide is discharged. This mixture may be sent to afuel pool. In step (8) a mixture of hydrogen and nitrogen is fed to thecolumn and bed and the effluent may be sent to the fuel pool. In step(9) the column and bed are pressurized to a high pressure again using amixture of hydrogen and nitrogen. During step (A) the pressure in thecolumn and bed is high. In steps (B) and (C) the pressure is reduced intotal by 5 to 10 bar a, and a remainder of hydrogen is discharged. Instep (D) the pressure in the column and bed is reduced to 1 to 5 bar a,and hydrogen and nitrogen are discharged. In step (E) hydrogen is fed tothe column and bed and the effluent may be sent to the fuel pool. Insteps (F) and (G) the column and bed are pressurized to a high pressureagain using hydrogen.

While the method has been described in terms of what are presentlyconsidered to be the most practical and preferred embodiments, it is tobe understood that the disclosure need not be limited to the disclosedembodiments. It is intended to cover various modifications, combinationsand similar arrangements included within the spirit and scope of theclaims, the scope of which should be accorded the broadestinterpretation so as to encompass all such modifications and similarstructures. The present disclosure includes any and all embodiments ofthe following claims.

It should also be understood that a variety of changes may be madewithout departing from the essence of the invention. Such changes arealso implicitly included in the description. They still fall within thescope of this invention. It should be understood that this disclosure isintended to yield a patent covering numerous aspects of the inventionboth independently and as an overall system and in both method andapparatus modes.

Any patents, publications, or other references mentioned in thisapplication for patent are hereby incorporated by reference. Inaddition, as to each term used, it should be understood that unless itsutilization in this application is inconsistent with suchinterpretation, common dictionary definitions should be understood asincorporated for each term and all definitions, alternative terms, andsynonyms such as contained in at least one of a standard technicaldictionary recognized by artisans.

1. A method for recovering carbon monoxide and carbon dioxide from a gasmixture comprising 5-50 vol % methane, 10-50 vol % carbon dioxide, 20-65vol % carbon monoxide, 10-40 vol % hydrogen and 10-55 vol % nitrogen,calculated on the total volume of the gas mixture, said methodcomprising, in sequence, the following steps: (1) feeding a gas mixturethrough a column comprising an adsorbent bed, said gas mixturecomprising 5-50 vol % methane, 10-50 vol % carbon dioxide, 20-65 vol %carbon monoxide, 10-40 vol % hydrogen and 10-55 vol % nitrogen,calculated on the total volume of the gas mixture, the adsorbent bedcomprising alumina, a carbon molecular sieve, silicalite, activatedcarbon, a zeolite, or mixtures thereof, with upon commencement of saidfeeding, the bed and column being pre-saturated and pre-pressurized to apressure in the range of 10 to 80 bar absolute (bar a) with purehydrogen, or with a mixture of hydrogen and nitrogen, and dischargingeffluent from the other end of said bed, and continuing said feeding andsaid discharging until a carbon monoxide comprising gas has reached atleast 45% of the length of the bed and has reached at most 80% of thelength of the bed, calculated from the end of the bed at which the gasmixture is being fed; and (2) ceasing the feeding of the gas mixturecomprising 5-50 vol % methane, 10-50 vol % carbon dioxide, 20-65 vol %carbon monoxide, 10-40 vol % hydrogen and 10-55 vol % nitrogen, andreducing the pressure in the column and the bed by about 5 to 25 bar a;and (3) rinsing the column and the adsorbent bed by feeding a gascomprising at least 70 vol % methane through the column and adsorbentbed, the column and bed being at a pressure in the range of 5 to 75 bara, and discharging effluent from the other end of said bed, andcontinuing said feeding and said discharging until at least 60% of thecarbon monoxide that was present in the bed at the commencement of thisrinsing step is discharged from the other end of said bed; (4)pressurizing the column and adsorbent bed by about 5 to 25 bar a byfeeding a gas comprising at least 70 vol % methane, (5) rinsing thecolumn and the adsorbent bed by feeding a gas comprising at least 70 vol% methane through the column and adsorbent bed, the column and bed beingat a pressure in the range of 30 to 80 bar a, and discharging effluentfrom the other end of said bed, and continuing said feeding and saiddischarging until an amount equal to at least 50% of the carbon dioxidethat was present at the commencement of this rinsing step is dischargedfrom the other end of said bed; and (6) ceasing the feeding of a gascomprising at least 70 vol % methane and reducing the pressure of thecolumn and adsorbent bed to a pressure in the range of 15 to 25 bar a;and (7) further reducing the pressure of the column and adsorbent bed toa pressure in the range of 1 to 5 bar a; and (8) rinsing the column andadsorbent bed by feeding a mixture of hydrogen and nitrogen through thecolumn and adsorbent bed the column and bed being at a pressure in therange of 1 to 5 bar a, and (9) pressurizing the column and adsorbent bedto a pressure in the range of 15 to 75 bar a by feeding a mixture ofhydrogen and nitrogen.
 2. (canceled)
 3. (canceled)
 4. A method accordingto claim 1, wherein repeated cycles of steps (1) to (9) are performed.5. A method according to claim 1, wherein at least two columns thatcomprise an adsorbent bed are used for steps (1) to (9).
 6. A methodaccording to claim 5, wherein the columns for steps (1) to (9) areconnected in parallel.
 7. A method according to claim 1, wherein atleast a part of the discharged effluent of step (3) is sent as feed to aFischer-Tropsch reaction.
 8. A method according to any one of thepreceding claims, wherein the column and adsorbent bed are rinsed instep (8) by feeding a gas comprising at least 95 vol % nitrogen throughthe column and adsorbent bed, followed by feeding a mixture of hydrogenand nitrogen through the column and adsorbent bed, the column and bedbeing at a pressure in the range of 1 to 5 bar a.
 9. A method accordingto claim 1, wherein a part of the effluent of step (2) is used asfeeding gas in step (8).
 10. A method according to claim 9, whereinadditionally a part of the effluent of step (1) is used as feeding gasin step (8).
 11. A method according to claim 1, wherein a part of theeffluent of step (1) is used as feeding gas in step (9).
 12. A methodaccording to claim 11, wherein additionally a part of the effluent ofstep (2) is used as feeding gas in step (9).
 13. A method according toclaim 1, wherein a scrubbed Fischer-Tropsch off-gas from aFischer-Tropsch reactor is fed to the column and adsorbent bed in step(1), and a part of the effluent of step (3) is sent as a recycle streamto the Fischer-Tropsch reactor that produced the off-gas.
 14. A methodaccording to claim 1, wherein a scrubbed Fischer-Tropsch off-gas from afirst Fischer-Tropsch reactor is used in step (1), and a part of theeffluent of step (3) is sent as a feed stream to a secondFischer-Tropsch reactor.
 15. A method for recovering carbon monoxide andcarbon dioxide from a gas mixture comprising 5-50 vol % methane, 10-50vol % carbon dioxide, 20-65 vol % carbon monoxide, 10-40 vol % hydrogenand 10-55 vol % nitrogen, calculated on the total volume of the gasmixture, said method comprising, in sequence, the following steps: (1)feeding a gas mixture through a column comprising an adsorbent bed, saidgas mixture comprising 5-50 vol % methane, 10-50 vol % carbon dioxide,20-65 vol % carbon monoxide, 10-40 vol % hydrogen and 10-55 vol %nitrogen, calculated on the total volume of the gas mixture, theadsorbent bed comprising alumina, a carbon molecular sieve, silicalite,activated carbon, a zeolite, or mixtures thereof, with upon commencementof said feeding, the bed and column being pre-saturated andpre-pressurized to a pressure in the range of 10 to 80 bar absolute (bara) with pure hydrogen, or with a mixture of hydrogen and nitrogen, anddischarging effluent from the other end of said bed, and continuing saidfeeding and said discharging until a carbon monoxide comprising gas hasreached at least 45% of the length of the bed and has reached at most80% of the length of the bed, calculated from the end of the bed atwhich the gas mixture is being fed; and (2) ceasing the feeding of thegas mixture comprising 5-50 vol % methane, 10-50 vol % carbon dioxide,20-65 vol % carbon monoxide, 10-40 vol % hydrogen and 10-55 vol %nitrogen, and reducing the pressure in the column and the bed by about 5to 25 bar a; and (3) rinsing the column and the adsorbent bed by feedinga gas comprising at least 70 vol % methane through the column andadsorbent bed, the column and bed being at a pressure in the range of 5to 75 bar a, and discharging effluent from the other end of said bed,and continuing said feeding and said discharging until at least 60% ofthe carbon monoxide that was present in the bed at the commencement ofthis rinsing step is discharged from the other end of said bed; (4)pressurizing the column and adsorbent bed by about 5 to 25 bar a byfeeding a gas comprising at least 70 vol % methane, (5) rinsing thecolumn and the adsorbent bed by feeding a gas comprising at least 70 vol% methane through the column and adsorbent bed, the column and bed beingat a pressure in the range of 30 to 80 bar a, and discharging effluentfrom the other end of said bed, and continuing said feeding and saiddischarging until an amount equal to at least 50% of the carbon dioxidethat was present at the commencement of this rinsing step is dischargedfrom the other end of said bed; and (6) ceasing the feeding of a gascomprising at least 70 vol % methane and reducing the pressure of thecolumn and adsorbent bed to a pressure in the range of 15 to 25 bar a;and (7) further reducing the pressure of the column and adsorbent bed toa pressure in the range of 1 to 5 bar a; and (8) rinsing the column andadsorbent bed by feeding a mixture of hydrogen and nitrogen through thecolumn and adsorbent bed the column and bed being at a pressure in therange of 1 to 5 bar a, and (9) pressurizing the column and adsorbent bedto a pressure in the range of 15 to 75 bar a by feeding a mixture ofhydrogen and nitrogen, whereby said method further comprises thefollowing steps: (A) feeding the effluent of step (1) through a columncomprising an adsorbent bed, the adsorbent bed comprising alumina, acarbon molecular sieve, silicalite, activated carbon, a zeolite, ormixtures thereof, with upon commencement of said feeding, the bed andcolumn being pre-saturated and pre-pressurized to a pressure in therange of 20 to 80 bar absolute (bar a) with a gas comprising more than95 volume % hydrogen, and discharging effluent from the other end ofsaid bed, and continuing said feeding and said discharging until anitrogen comprising gas has reached at least 45% of the length of thebed and has reached at most 80% of the length of the bed, calculatedfrom the end of the bed at which the gas mixture is being fed; and (B)ceasing the feeding of the effluent of step (1), and reducing thepressure in the column and the bed by about 2 to 5 bar a; and (C)further reducing the pressure in the column and the bed by about 2 to 5bar a; and (D) reducing the pressure of the column and adsorbent bed toa pressure in the range of 1 to 5 bar a; and (E) rinsing the column andadsorbent bed by feeding a gas comprising more than 95 volume % hydrogenthrough the column and adsorbent bed the column and bed being at apressure in the range of 1 to 5 bar a, and (F) pressurizing the columnand adsorbent bed to a pressure in the range of 5 to 50 bar a by feedinga gas comprising more than 95 volume % hydrogen, and (G) furtherpressurizing the column and adsorbent bed to a pressure in the range of15 to 75 bar a by feeding a gas comprising more than 95 volume %hydrogen.
 16. A method according to claim 15, wherein repeated cycles ofsteps (1) to (9) are performed and repeated cycles of steps (A) to (G)are performed.
 17. A method according to claim 15, wherein at least twocolumns that comprise an adsorbent bed are used for steps (1) to (9) andat least two columns that comprise an adsorbent bed are used for steps(A) to (G).
 18. A method according to claim 15, wherein the columns forsteps (1) to (9) are connected in parallel, and the columns for steps(A) to (G) are connected in parallel.
 19. A method according to claim15, wherein at least a part of the discharged effluent of step (3) issent as feed to a Fischer-Tropsch reaction, and at least a part of thedischarged effluent of step (A) is sent, separately or in combinationwith effluent of step (3), as a feed to a Fischer-Tropsch reaction. 20.A method according to claim 15, wherein a part of the effluent of step(2) is used as feeding gas in step (8), and at least a part of thedischarged effluent of step (D) and/or of step (E) is used, separatelyor in combination with effluent of step (2), as a feeding gas in step(8).